There is one planet that still fascinates and teases scientists, mainly because it doesn't have an atmosphere to obscure observation, yet it is not big enough for sufficiently accurate telescopic observation. The fact that it is also very close to the sun also makes it difficult 5 for astronomers.

However, space telescopes have told us a lot more about Mercury. It is rather similar to Earth's moon, and to Mars. There are mountainous areas dotted with craters and large valleys which are uncratered. 10 The moon's valleys have fewer cracks and the ridges are smoother. Mercury's valleys are filled with volcanic rocks, similar to on the moon, yet there is no evidence of volcanoes, even extinct ones, on Mercury, as there is on both Mars and the moon, and of course here on Earth.15So, scientists presume the valleys on Mercury were caused by different things, that is to say not by volcanoes, but by very large meteorites, which also caused the cracks in the terrain and left the ridges rough and uneven. More and better photos of Mercury are needed to prove 20 what are at present at best only hypotheses.

1. What is the topic of this passage?(A) The planet, Mercury.(B) Observation of Planets.(C) The earth's moon.(D) Astronomers' difficulties.2. Why does one planet still tease scientists?(A) Because it has an atmosphere which obscures observation.(B) Because it is so small.(C) Because it is too close to the sun.(D) Because it is difficult for space probes to get close to.3. What is true about Mercury but not true about the moon or Mars?(A) The valleys are filled with volcanic rocks.(B) There are hilly regions.(C) The valleys are uncratered.(D) There are no volcanoes.4. What does the author imply space telescopes can do better than land-based telescopes?(A) Show the volcanoes on Mercury.(B) Show more of the planet, Mercury.(C) Show what caused Mercury's valleys.(D) Show the meteorite which hit Mercury.5. What can we infer from the passage that astronomers still need to get more detailed observation of Mercury?(A) Larger space telescopes.(B) More hypotheses.(C) Better or more space telescopes.(D) More ways to prove hypotheses.